ECHINODERMA TA—CRINOIDEA . 5 47 



152. R. stellaris Billings. Ordovicic. 

 Arms extremely short and tapering rapidly. Interbrachial de- 

 pressions paved by numerous irregular pieces with a slightly stellate 

 surface. (Type of genus.) 



Trenton of Ottawa. 



LIV. Thysanocrinus Hall. 



Calyx subglobose or urn-shaped. IB 5 and small, often hidden 

 by the column. B 5, four of them equal and angular above, the 

 fifth truncated and supporting a large 

 anal plate. The rays marked by a 

 ridge; surface of plates otherwise 

 smooth or ornamented. Costals two. 

 Arms 10 or 20, biserial. Pinnules 

 long. Column round or obtusely 5- 

 angled (Fig. 1876). 



Siluric of America, England and 

 Sweden. ^^ Cy^ 



153. T. inornatus (Hall). Siluric. ^"^- "*7*- ^^-«~- (^f- 



^^ ^ ^ terWeller.) 



Calyx urn-shaped. Cross section at 

 top of costals pentagonal and across distichals decagonal. B each 

 with a node and hence forming a 5-lobed rim around the column. 

 Ill-defined radiating ridges follow the median line of the rays and 

 also branch laterally. Interbrachial spaces deeply depressed so as 

 to give the calyx at the arm bases a prominently lobed aspect. 

 Anus at end of a conspicuous ridge of anal plates. Tegmen 

 depressed. 



Niagaran of Indiana and Wisconsin. 



154. T. occidentalis Hall. Siluric. 

 Larger than T. inornatus and with no anal ridge. Ornamenta- 

 tion consisting of ridges forming a pentagon around the column 

 from whose angles pass other ridges spreading over the B and R 

 and continued up the brachials of the calyx. 



Niagaran of Indiana. 



155. T. pentangularis (Hall). Siluric. 

 Calyx inverted bell-shaped, with five-lobed rim around the base, 



each node of the basals giving rise to two ridges which extend up 



